Sounds by Night Can Strengthen Memory by Day

Many people swear that a good nap can refresh the memory; indeed, research confirms that what we learn while waking can be reinforced during sleep. Now a team from Northwestern University has demonstrated that one way napping strengthens individual memories is if the right “cues” are provided.

In a study published online on Nov. 20 in Science, volunteers tried to memorize the specific locations of 50 images in a grid on a computer screen. For each picture, they heard a characteristic sound, such as a meow for a cat or a whistle for a kettle. They then napped for up to 90 minutes.

Unbeknownst to the sleepers, researchers played some of the sounds during the nap—a different set of 25 sounds for each person. Upon awakening, participants repeated the memory test. Of the 12 subjects, 10 were better able to locate the “cued” images (those for which they’d heard the sounds while asleep) than the “uncued” ones. EEGs showed that the sounds elicited brain activity during sleep, suggesting that memories were being rehearsed at that time.

The new study clarifies previous research by demonstrating that memories accompanied by a sensory cue are strengthened. “It’s known that sleep can improve memory in general, but our findings suggest that memory processing during sleep can be highly specific,” says lead author Ken Paller of Northwestern University.

The research picks up where an earlier study left off. Reporting in the March 9, 2007, Science, Jan Born and colleagues at the University of Lubeck, Germany, improved their subjects’ performance in a game of concentration using one of the best-known memory enhancers—scent. Participants memorized the locations of 15 pairs of cards on a screen while breathing a rose fragrance. Subjects given the rose scent in their sleep that night scored better on the test next morning—getting 97 percent of card pairs correct, compared with 86 percent for the “unscented” subjects.

Born says that because the human brain’s ability to discriminate among smells is not well developed, scent is a less precise cue than sound. By choosing scent rather than sounds, his group sought to avoid the chance that participants hearing sounds learned the tasks better because the sounds had woken them up. But in the new study, EEG recordings showed that the sounds played were soft enough that they did not disturb participants’ sleep.

“The study by Paller and colleagues is a good step forward,” Born says. “To study sleep and memory more precisely, you have to move to the auditory system, which is much better deveoped in humans.”

The study also challenges the argument that sleep does not strengthen memory through any active process, says Bob Stickgold of the Center for Sleep and Cognition at Beth Israel Deaconess Medical Center in Boston. “Some researchers still believe recent memories are merely protected from outside interference during sleep because nothing else is happening—it’s dark, your eyes are shut, you’re not thinking,” Stickgold says. “If this were true, then all of the subjects’ memories would have improved, not just the ones linked to their sounds.”

Stickgold is impressed by the fact that the sound cue was not necessary initially to learn the image’s location; the sound merely played when the picture appeared. Even so, during sleep the sound by itself was enough to strengthen the connection between the visual and spatial components of the memory.

“It’s clear the brain can reactivate a complex memory even when given just one piece of it,” Stickgold says. “We know that happens normally. The question now is whether it happens more often, or more efficiently, during sleep.”

“Our findings open up a whole range of other things we’d like to know,” Paller agrees. His team has begun a new study using functional imaging to explore the parts of the brain involved. Animal research suggests that in the hippocampus—a memory nexus in the brain—neurons that fire when a new task is learned will fire in the same order after the animal goes to sleep. In the work by Born and colleagues, fMRI showed that re-exposure to the rose scent during sleep re-activated the same hippocampal network that was at work during the waking task—but to a greater degree.

Paller emphasizes that although sleep may contribute to memory, he does not advocate napping in place of studying. “Our finding doesn’t show that sleep is the best time to learn, only that memories are processed during sleep as well as waking.”